The present invention generally relates to laser processing systems as disclosed, for example, in U.S. Pat. Nos. 6,559,412; 5,812,569; 5,998,759; 6,339,604; 6,727,458; and 6,541,731 as well as U.S. Patent Application Publication Number No. 2002/0167581, each of which is assigned to the assignee of the present invention, and each of which is hereby incorporated by reference in its entirety. Laser based micromachining for memory repair of integrated circuits, for example, is disclosed in LIA Handbook of Laser Materials Processing, by John Ready, Laser Institute of America, Mongolia Publishing Co., Inc. (2001), chapter 19.
The above mentioned laser processing systems are generally used for a variety of micromachining tasks including, for example, blowing links of integrated circuits to repair memory devices. In such laser processing systems, a laser source generates pulses that are positioned and focused on a workpiece to perform the processing task. For high-speed precision processing of regularly spaced targets, such as one dimensional (1D) or two dimensional (2D) arrays of links for memory repair, lasers are pulsed at a pulse rate that approximately corresponds to the link pitch, and an optical switch is used to select pulses to blow only selected links. Since the laser source is operated at a near constant rate, the pulse energy is consistent which is desired for processing capability. Pulses may be generated during motion of a target relative to a laser pulse. Conventional laser processing systems such as those identified above generally involve pulse generation with various laser sources, pulse selection, and beam delivery for processing links or similar microscopic structures.
One type of optical switch used in the optical path between the laser source and the focus lens (e.g., external to the laser cavity) to select pulses is an acousto-optic modulator (AOM). This type of switch may be used to attenuate pulses and to set the output pulse energy for the selected pulses. The pulse energy may be set according to the processing parameters, or to a low energy level for beam alignment.
The optical rise time of AOMs is generally slower than Electro-Optic-Modulators (EOMs, Pockel Cells). The ease of use however, continues to make the AOM an attractive option. Although development of integrated electro-optic switches with half-wave voltages of a few volts and nanosecond or faster rise is progressing, AOMs are a well established alternative for most micromachining applications.
In some micromachining applications including memory repair, multiple pulses may be delivered to the workpiece in a group of closely spaced pulses. A suitable single laser, for example a mode-locked laser or semiconductor diode, may produce a rapid burst of pulses. Alternatively, multiple closely spaced pulses may be generated with a system in which outputs of two or more laser sources are combined.
It is desirable to provide pulse selection within a group of multiple pulses for processing and alignment. Intermittent pulsing, however, may generally lead to unstable pulse energy. Each source may be continuously pulsed and modulated by a modulator for each laser source. This however, is not a preferred solution. Pulse selection with a single modulator is desirable, but close spacing of less than about 100 ns within a group is generally beyond the limits of most acousto-optic modulators. There is a need therefore, for pulse selection within a group of closely space pulses that provides stable laser operation using a single modulator.